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  <div class="section" id="fix-efield-command">
<span id="index-0"></span><h1>fix efield command<a class="headerlink" href="#fix-efield-command" title="Permalink to this headline">¶</a></h1>
<div class="section" id="syntax">
<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
<div class="highlight-python"><div class="highlight"><pre>fix ID group-ID efield ex ey ez keyword value ...
</pre></div>
</div>
<ul class="simple">
<li>ID, group-ID are documented in <a class="reference internal" href="fix.html"><em>fix</em></a> command</li>
<li>efield = style name of this fix command</li>
<li>ex,ey,ez = E-field component values (electric field units)</li>
<li>any of ex,ey,ez can be a variable (see below)</li>
<li>zero or more keyword/value pairs may be appended to args</li>
<li>keyword = <em>region</em> or <em>energy</em></li>
</ul>
<pre class="literal-block">
<em>region</em> value = region-ID
  region-ID = ID of region atoms must be in to have added force
<em>energy</em> value = v_name
  v_name = variable with name that calculates the potential energy of each atom in the added E-field
</pre>
</div>
<div class="section" id="examples">
<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
<div class="highlight-python"><div class="highlight"><pre>fix kick external-field efield 1.0 0.0 0.0
fix kick external-field efield 0.0 0.0 v_oscillate
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
<p>Add a force F = qE to each charged atom in the group due to an
external electric field being applied to the system.  If the system
contains point-dipoles, also add a torque on the dipoles due to the
external electric field.</p>
<p>For charges, any of the 3 quantities defining the E-field components
can be specified as an equal-style or atom-style
<a class="reference internal" href="variable.html"><em>variable</em></a>, namely <em>ex</em>, <em>ey</em>, <em>ez</em>.  If the value is a
variable, it should be specified as v_name, where name is the variable
name.  In this case, the variable will be evaluated each timestep, and
its value used to determine the E-field component.</p>
<p>For point-dipoles, equal-style variables can be used, but atom-style
variables are not currently supported, since they imply a spatial
gradient in the electric field which means additional terms with
gradients of the field are required for the force and torque on
dipoles.</p>
<p>Equal-style variables can specify formulas with various mathematical
functions, and include <a class="reference internal" href="thermo_style.html"><em>thermo_style</em></a> command
keywords for the simulation box parameters and timestep and elapsed
time.  Thus it is easy to specify a time-dependent E-field.</p>
<p>Atom-style variables can specify the same formulas as equal-style
variables but can also include per-atom values, such as atom
coordinates.  Thus it is easy to specify a spatially-dependent E-field
with optional time-dependence as well.</p>
<p>If the <em>region</em> keyword is used, the atom must also be in the
specified geometric <a class="reference internal" href="region.html"><em>region</em></a> in order to have force added
to it.</p>
<hr class="docutils" />
<p>Adding a force or torque to atoms implies a change in their potential
energy as they move or rotate due to the applied E-field.</p>
<p>For dynamics via the &#8220;run&#8221; command, this energy can be optionally
added to the system&#8217;s potential energy for thermodynamic output (see
below).  For energy minimization via the &#8220;minimize&#8221; command, this
energy must be added to the system&#8217;s potential energy to formulate a
self-consistent minimization problem (see below).</p>
<p>The <em>energy</em> keyword is not allowed if the added field is a constant
vector (ex,ey,ez), with all components defined as numeric constants
and not as variables.  This is because LIGGGHTS(R)-PUBLIC can compute the energy
for each charged particle directly as E = -x dot qE = -q (x*ex + y*ey
+ z*ez), so that -Grad(E) = F.  Similarly for point-dipole particles
the energy can be computed as E = -mu dot E = -(mux*ex + muy*ey +
muz*ez).</p>
<p>The <em>energy</em> keyword is optional if the added force is defined with
one or more variables, and if you are performing dynamics via the
<a class="reference internal" href="run.html"><em>run</em></a> command.  If the keyword is not used, LIGGGHTS(R)-PUBLIC will set
the energy to 0.0, which is typically fine for dynamics.</p>
<p>The <em>energy</em> keyword is required if the added force is defined with
one or more variables, and you are performing energy minimization via
the &#8220;minimize&#8221; command for charged particles.  It is not required for
point-dipoles, but a warning is issued since the minimizer in LIGGGHTS(R)-PUBLIC
does not rotate dipoles, so you should not expect to be able to
minimize the orientation of dipoles in an applied electric field.</p>
<p>The <em>energy</em> keyword specifies the name of an atom-style
<a class="reference internal" href="variable.html"><em>variable</em></a> which is used to compute the energy of each
atom as function of its position.  Like variables used for <em>ex</em>, <em>ey</em>,
<em>ez</em>, the energy variable is specified as v_name, where name is the
variable name.</p>
<p>Note that when the <em>energy</em> keyword is used during an energy
minimization, you must insure that the formula defined for the
atom-style <a class="reference internal" href="variable.html"><em>variable</em></a> is consistent with the force
variable formulas, i.e. that -Grad(E) = F.  For example, if the force
due to the electric field were a spring-like F = kx, then the energy
formula should be E = -0.5kx^2.  If you don&#8217;t do this correctly, the
minimization will not converge properly.</p>
</div>
<hr class="docutils" />
<div class="section" id="restart-fix-modify-output-run-start-stop-minimize-info">
<h2>Restart, fix_modify, output, run start/stop, minimize info<a class="headerlink" href="#restart-fix-modify-output-run-start-stop-minimize-info" title="Permalink to this headline">¶</a></h2>
<p>No information about this fix is written to <a class="reference internal" href="restart.html"><em>binary restart files</em></a>.</p>
<p>The <a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a> <em>energy</em> option is supported by this
fix to add the potential &#8220;energy&#8221; inferred by the added force due to
the electric field to the system&#8217;s potential energy as part of
<a class="reference internal" href="thermo_style.html"><em>thermodynamic output</em></a>.  This is a fictitious
quantity but is needed so that the <code class="xref doc docutils literal"><span class="pre">minimize</span></code> command
can include the forces added by this fix in a consistent manner.
I.e. there is a decrease in potential energy when atoms move in the
direction of the added force due to the electric field.</p>
<p>This fix computes a global scalar and a global 3-vector of forces,
which can be accessed by various <a class="reference internal" href="Section_howto.html#howto-8"><span>output commands</span></a>.  The scalar is the potential
energy discussed above.  The vector is the total force added to the
group of atoms.  The scalar and vector values calculated by this fix
are &#8220;extensive&#8221;.</p>
<p>No parameter of this fix can be used with the <em>start/stop</em> keywords of
the <a class="reference internal" href="run.html"><em>run</em></a> command.</p>
<p>The forces due to this fix are imposed during an energy minimization,
invoked by the <code class="xref doc docutils literal"><span class="pre">minimize</span></code> command.  You should not
specify force components with a variable that has time-dependence for
use with a minimizer, since the minimizer increments the timestep as
the iteration count during the minimization.</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">If you want the fictitious potential energy associated
with the added forces to be included in the total potential energy of
the system (the quantity being minimized), you MUST enable the
<a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a> <em>energy</em> option for this fix.</p>
</div>
</div>
<div class="section" id="restrictions">
<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
<blockquote>
<div>none</div></blockquote>
</div>
<div class="section" id="related-commands">
<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
<p><a class="reference internal" href="fix_addforce.html"><em>fix addforce</em></a></p>
<p><strong>Default:</strong> none</p>
</div>
</div>


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